(1) Field of the Invention
The present invention relates to a color cathode ray tube (CRT) used as a monitor, a television receiver, or the like, and particularly relates to a means that corrects cross-misconvergence occurring in a horizontal strip in the central part of both the upper and lower halves of a fluorescent screen of the color CRT.
(2) Related Art
When blue and red rasters are projected onto a fluorescent screen of a conventional color CRT provided with an in-line type electron gun, so-called "cross-misconvergence" occurs as shown in FIG. 1. This cross-misconvergence takes place due to a delicate interrelationship between a distorted distribution of a magnetic field generated by a deflection device (or, a deflection yoke) and a shape of an inner surface of a front panel of the color CRT.
The cross-misconvergence refers to a phenomenon in which blue and red rasters vertically deviate from each other in a horizontal strip in the central part of both the upper and lower halves of the effective display region of a fluorescent screen 50. As shown in FIG. 1, the upper half includes a first quadrant indicated as (I) and a second quadrant indicated as (II), while the lower half includes a third quadrant as (III) and a fourth quadrant as (IV). Hereinafter, each horizontal strip in the central parts of the quadrants is referred to as the "central strip."
As shown in FIG. 1, blue rasters 1B to 4B and red rasters 1R to 4R are projected in different slanting directions. In the quadrants (I) and (III), blue rasters 1B and 3B drawn in dashed lines are located respectively above red rasters 1R and 3R drawn in solid lines. In the quadrants (II) and (IV), blue rasters 2B and 4B are located respectively below red rasters 2R and 4R.
Here, Japanese Laid-Open Patent No. 64-84549 discloses a method to reduce the occurrence of such cross-misconvergence. This method is specifically explained as follows. Vertical deflection coils of a deflection device include a pair of coils for generating magnetic fields distorted in a pincushion and a pair of coils for generating magnetic fields distorted in a barrel. Two diodes in parallel, with their polarities being opposite, are connected in series to the pair of coils generating the pincushion magnetic fields. With this construction, the magnetic field is switched between the pincushion and barrel magnetic fields at a timing at which electron beams are deflected to the central strip, in order that the stated cross-misconvergence can be prevented.
Meanwhile, Japanese Laid-Open Utility Model No. 63-80756 discloses another method of correcting cross-misconvergence. In the disclosure, at least four permanent magnets are set around a front rim of a bobbin of a deflection device, each permanent magnet having the magnetic poles parallel to the axial direction of the bobbin and being set on an extended diagonal line of the bobbin. By means of these permanent magnets, raster distortion and cross-misconvergence in the central strips can be simultaneously corrected.
Although the cross-misconvergence is reduced using the method disclosed in the former reference, the linearity of vertical rasters is deteriorated due to sudden switches from the barrel magnetic field to the pincushion magnetic field. This leads to deterioration in images displayed on the fluorescent screen of the CRT.
Using the technique disclosed in the latter reference, meanwhile, because of the permanent magnets, cross-misconvergence next occurs in areas around the horizontal axis of the fluorescent screen, where it has never occurred. Thus, although the cross-misconvergence occurring in the central strips is corrected, convergence quality of the entire screen cannot be improved.